A duplexer may be provided to a conventional radio communication device in which an antenna is shared between transmission and reception. In other words, when the frequency differs between a transmission signal and a reception signal, connection of the duplexer to the antenna enables electrical separation between a transmission path and a reception path in the radio communication device. Accordingly, the transmission signal does not interfere with the reception signal, which makes it possible to inhibit the reception quality from lowering.
A duplexer, however, mainly consists of a filter and it is difficult to completely prevent the transmission signal from leaking into the reception path. For this reason, intermodulation of the leaking transmission signal and the received interference signal may be caused in the reception path and the resultant intermodulation distortion may lower reception quality. In other words, when the frequency of a transmission signal and the frequency of an interference signal differ from each other, the intermodulation between these signals causes intermodulation distortion at a given frequency. When the frequency of the intermodulation distortion is contained in the frequency band of the reception signal, the intermodulation distortion hinders demodulation and decoding of the reception signal. Then, the accuracy of the reception data obtained from the reception signal lowers. For this reason, examinations have been performed on approximately reproducing the intermodulation distortion on the basis of the transmission signal and the interference signal and offsetting the intermodulation distortion according to the reproduced reproduction signal.
Patent Document 1: Japanese National Publication of International Patent Application No. 2010-500831
In recent years, the signal bandwidth used for radio communications have been increasing and multicarrier transmission in which signals are transmitted with multiple carriers at different frequencies have been practically used. In multicarrier transmission, because multiple transmission signals having different frequencies are transmitted, intermodulation between the transmission signals may cause intermodulation distortion.
Furthermore, the duplexer, the antenna, and a cable that connects the duplexer and the antenna to each other are a linear circuit and, normally, intermodulation distortion does not occur in these parts; however, intermodulation distortion due to the transmission signal may occur in the linear circuit because of an internal factor, such as erosion of the material or contamination of the connection surfaces between different types of metal or an external factor, such as a magnetic field or oscillation. The noted intermodulation distortion may be referred to as passive intermodulation (PIM). While the power of PIM is smaller than the power of a transmission signal, the power of the reception signal is generally smaller than the power of the transmission signal and therefore it is not allowed to ignore the influence of the PIM on the reception signal.
While, with respect to the reception signal to which intermodulation distortion, such as PIM, is added, the distortion may be canceled by using a reproduction signal obtained by approximately reproducing the intermodulation as described above, there is a problem in that it is difficult to improve the reception quality depending on the accuracy of the reproduction signal. In other words, it is difficult to completely reproduce the intermodulation distortion added to the reception signal and therefore, when the intermodulation distortion is canceled by adding the reproduction signal to the reception signal, part of the reproduction signal may be a noise. As a result, the noise added to the reception signal may increase, which may lower the reception quality.
Furthermore, when the intermodulation distortion in the reception signal is canceled, the intermodulation distortion that occurs due to the intermodulation between transmission signals is emitted while being added to the transmission signal. For this reason, there is a problem in that the intermodulation distortion may be a spurious emission.